Safety trocar

ABSTRACT

A safety trocar is provided in which the cannula is reciprocally mounted to the cannula housing and reciprocates into the cannula housing when a proximally directed force is applied thereto. The piercing tip is maintained in the exposed position by a latching mechanism associated with the obturator shaft, and is automatically withdrawn into the cannula under the force of a spring when the proximal force is removed from the cannula, the cannula thus advancing distally and releasing the latching mechanism. Penetration force is maintained at a minimum, and safe and efficacious trocar entry is facilitated.

BACKGROUND

[0001] 1. Technical Field

[0002] This invention relates to a surgical trocar and more particularlyto a safety trocar in which the sharp cutting tip retracts into thecannula so as to minimize the likelihood of inadvertent injury toviscera and other internal tissue.

[0003] 2. Background of Related Art

[0004] Trocars are sharp pointed surgical instruments used to puncture abody cavity. Trocars are generally adapted to be used together with atubular trocar sleeve or cannula. Once the body cavity has beenpunctured by the trocar, the sharp trocar is removed from the cannula,thereby leaving the cannula extending into the body cavity. Endoscopicsurgical procedures are then performed through the cannula withaccessory instrumentation such as laparoscopes, dissectors, graspers,etc.

[0005] Commercially available safety trocars include a spring-loadedsafety shield which is adapted to cover the trocars tip once the bodycavity has been entered so as to provide an increased level ofprotection to internal structures from puncture of laceration. Forexample, U.S. Pat. No. 4,601,710 to Moll describes a trocar assemblywhich consists of two subassemblies: a trocar subassembly which includesa sharp-tipped trocar and a spring-loaded tubular safety shieldpositioned therearound, and a cannula subassembly.

[0006] When ready for use, the trocar and safety shield of the trocarsubassembly are inserted through the cannula. The safety shield isinitially in its distal-most position covering the trocar tip. Exertionof pressure against the skin with the trocar causes the shield to bepushed rearwardly against the spring to expose the piercing tip of thetrocar. The tip penetrates the skin and underlying tissue with continuedpressure. Once the tip has penetrated through the wall and has enteredthe cavity, the force against the front end of the shield ceases and theshield is automatically moved back to its distally extended position.Viscera and other internal tissue are thus protected from contact withthe sharp piercing tip and potential damage therefrom.

[0007] An article entitled “Needle for the Puncture and Lavage of theAbdominal Cavity” authored by F. S. Subairov discloses a safety devicefor puncturing the abdominal cavity which consists of a hollow tube, astylet and a spring. The spring is soldered to the stylet and threadedinto the rear of the hollow tube. The distal end of the stylet isexposed from the hollow tube by pressing the stylet toward the tube,thereby compressing the spring. Once the stylet and tube enter a bodycavity, the tube is advanced under spring force to cover the distal endof the stylet. A similar device is disclosed in EP 350,291 (see FIGS.1-4).

[0008] U.S. Pat. No. 4,535,773 to Yoon suggests several alternativesafety trocar designs. In one embodiment (see FIGS. 22-28), aspring-loaded blunt probe is provided within the trocar shaft, as withconventional Verres needles. The blunt probe is adapted to reciprocallyslide through an aperture in the trocar tip such that when the trocartip enters a body cavity, the blunt probe springs distally forwardthrough the aperture to prevent contact between the trocar tip and bodyorgans. In a second embodiment (see FIGS. 33-36), pressure sensors ortransducers are fitted into the trocar blade surfaces and the distal endof the cannula. Sets of electrical leads run through the trocar shaftand communicate with an alarm network in the proximal portion of thedevice. A further modification is suggested in which the trocar shaft isinitially manually extended and maintained in its extended position by adetent which protrudes through a hole in the surrounding tubularstructure. The hole aligns with a solenoid socket. When the instrumentis fully assembled and the trocar tip is forced through a body wall, theelectrical leads running through the trocar shaft send electricalsignals to the solenoid which, at the appropriate instant, forces thedetent from the hole, allowing the trocar tip to withdraw into thecannula. Additional mechanisms for effecting withdrawal of cuttingimplements are also known. See, e.g., U.S. Pat. No. 4,375,815 to Bums;U.S. Pat. No. 3,657,812 to Lee; and U.S. Pat. No. 3,030,959 to Grunert.

SUMMARY

[0009] It has now been found that an improved safety trocar may beprovided which includes:

[0010] (a) a cannula assembly comprising a cannula and a cannulahousing;

[0011] (b) a trocar assembly comprising a sharp trocar tip, an obturatorshaft, and a trocar housing;

[0012] (c) means associated with the obturator shaft which releasablymaintains the trocar tip in an extended position;

[0013] (d) means associated with the cannula assembly for releasing thereleasable obturator means; and

[0014] (e) biasing means for retracting the trocar tip from the extendedposition to a retracted position in response to release of thereleasable obturator means.

[0015] The safety trocar of the present invention is adapted to be armedby the surgeon immediately prior to use. Arming may be accomplished byadvancing a button which extends through the trocar housing, bycompressing the trocar housing toward the cannula housing, or by likemeans. Once armed, the trocar tip releasably protrudes beyond the distalend of the cannula.

[0016] As the surgeon presses the trocar, and more particularly thetrocar tip, against the body wall of a patient, an incision into andthrough the body wall is begun. With continued pressure by the surgeon,the distal end of the cannula comes into contact with the body wall. Theinitial counterforce exerted by the body wall against the cannula causesa mechanism associated with the cannula to position the obturator shaft(together with the cutting tip) for immediate retraction upon entry ofdistal end of the cannula into the body cavity. Thus, removal of thecounterforce from the distal end of the cannula, e.g., upon entering thebody cavity, results in immediate and automatic withdrawal of the trocartip into the cannula under the force of a biasing means, e.g., a spring.

[0017] In a preferred embodiment of the trocar, a latch is associatedwith the obturator shaft to which the trocar tip is mounted, the latchbeing biased radially outward and being adapted to engage an internalshelf formed in the cannula when the trocar is armed. The cannula isreciprocally mounted to the cannula housing and biased, e.g., by acompression spring, distally relative to the cannula housing. As thetrocar tip enters the body cavity and the body wall exerts force on thedistal end of the cannula, the cannula reciprocates proximally into thecannula housing. This cannula reciprocation repositions the cannula'sinternal shelf relative to the latch such that, upon distal movement ofthe cannula upon entry into the body cavity, the latch is released fromengagement with the internal shelf. A spring which was loaded uponarming the trocar is thus free to immediately retract the trocar tipinto the cannula. In a particularly preferred trocar embodiment,abutment means are provided on the exterior of the cannula toward itsdistal end to facilitate reciprocation thereof through contact with thebody wall.

[0018] The trocar of the invention is also designed to permit manualretraction or disarming of the cutting tip, if so desired. This isaccomplished by manually reciprocating the cannula housing andreleasing, thereby disengaging the latch from the internal shelf. Thetrocar is also typically provided with an indicator which signals thesurgeon as to whether the trocar is armed or disarmed. For example, therelative position of the button used to arm the trocar may be calibratedor indexed to communicate the trocar tip position or a window may beprovided through which a trocar tip position indicator is visible.

[0019] The trocar of the present invention provides a safe andefficacious means for gaining access to body cavities to permitminimally-invasive diagnostic and surgical procedures to beaccomplished. The trocar is equipped with a reliable mechanism foreffectuating immediate, automatic retraction of the cutting tip into thecannula. Penetration force is kept to a minimum through the uniqueinternal mechanism for releasably maintaining the trocar tip in thearmed position.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The present invention will become apparent from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

[0021]FIG. 1 is a side view of a trocar of the present invention in anarmed configuration;

[0022]FIG. 2 is a top view of the trocar assembly of FIG. 1, partiallyin cross section;

[0023]FIG. 3 is an exploded view of a spring assembly;

[0024]FIG. 4 is a front view of a top trocar housing;

[0025]FIGS. 5 and 6 are plan and side views, respectively, of aspring-retaining plate;

[0026]FIGS. 7 and 8 are partial side views of the cannula;

[0027]FIG. 9 is a plan view of a lower cannula housing;

[0028]FIG. 10 is a sectional side view of the cannula housing of FIG. 9taken along line 10—10;

[0029]FIG. 11 is a bottom view of a trocar housing;

[0030]FIG. 12 is a side view of a shelf insert;

[0031]FIG. 13 is a front view of the shelf insert of FIG. 12;

[0032]FIG. 14 is a side view of a latch;

[0033]FIG. 15 is a side view of a latch release finger;

[0034]FIG. 16 is a side view, partially in section, of a portion of acannula;

[0035]FIG. 17 is a side view of a latch subassembly.

[0036]FIG. 18 is a top view, partially in section, of the proximal endof an unarmed trocar of the present invention; and

[0037] FIGS. 19-21 are top views, partially in section, of the proximalend of an armed trocar of the present invention showing a sequence ofpositions which culminate in retraction of the trocar tip.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS

[0038] Referring to FIG. 1, trocar 10 is shown in its fully assembledcondition with cutting tip 12 extending from cannula 14. Trocar 10includes a trocar assembly 16 and a cannula assembly 18. Thelongitudinally extending or endoscopic portion of trocar assembly 16which extends from trocar housing 20 is shielded from view in FIG. 1 bycannula housing 21 and cannula 14, except for extended cutting tip 12. Acircumferential abutment surface 15 is provided toward the distal end ofcannula 14.

[0039] Referring now to FIGS. 2 and 3, pyramidal cutting tip 12 isrotatably mounted to obturator shaft 22 at swivel joint 24. Acylindrical channel 26 is formed in the proximal end of obturator shaft22. A tension spring 28 (see FIGS. 1 and 3) is positioned within channel26 and anchored to obturator shaft 22 at distal extension 30 by pin 32.An anchor pin 34 which is adapted to mount to rear wall 36 of trocarhousing 20 includes an aperture 35 which cooperates with proximal arm 37to anchor tension spring 28 to trocar housing 20.

[0040] A flange 38 is formed at the proximal end of obturator shaft 22encircling tension spring 28. Flange 38 forms a half-moon shaped button40 which slides within slot 42 in the upper face 44 of trocar housing20. Movement of button 40 within slot 42 moves obturator shaft 22 andcutting tip 12 relative to trocar housing 20. Tension spring 28 biasesobturator shaft 22 and its accompanying structure proximally, such thatflange 38 rests against rear face 46 of slot 42 absent externalinfluences.

[0041] Referring to FIG. 4, top housing 48 of trocar housing 20 isshown. Top housing 48 includes mounting legs 50 for attachment to abottom housing 90 (FIG. 11). Upper face 44 includes a concave region 52in the base of which is formed slot 42. As most clearly seen in FIG. 1,button 40 is sized and dimensioned to conform to and slide withinconcave region 52 while extending slightly above upper face 44 of trocarhousing 20. The cooperation between concave region 52 and button 40facilitates unimpeded movement of button 40, particularly uponretraction of trocar tip 12 as discussed below, without sacrificingconvenient thumb access to button 40 for arming of trocar 10 by thesurgeon.

[0042] Turning to cannula assembly 18, cannula 14 defines a tubularlumen and is reciprocally mounted to cannula housing 21. Referring toFIGS. 2 and 5-8, a spring-retaining plate 54 is mounted to flange 56 atthe proximal end of cannula 14, e.g., by adhesive or welding, withcentral aperture 58 in plate 54 aligned with the lumen through cannula14. Aperture 58 is sized to accommodate unencumbered passage ofobturator shaft 22 and includes oppositely directed extension arms 62,each arm 62 having an aperture 64. Apertures 64 receive and frictionallyengage cylindrical pins 66. Aperture extension 60 is formed in oneextension arm 62 allowing passage of latch subassembly 147 therethrough,as described below.

[0043] Referring again to FIG. 2, cannula housing 21 receivesspring-retaining plate 54 with cylindrical pins 66 facing proximally.Compression springs 68 are positioned against extension arms 62 andaround pins 66. Pins 66 thus act to position and support springs 68.Chambers 70 are formed in cannula housing 21 to receive and capture theopposite ends of compression springs 68. Reciprocation of cannula 14into cannula housing 21 causes proximal movement of plate 54 whichcompresses springs 68 within chambers 70, thereby biasing cannula 14 inthe distal direction.

[0044] Returning to FIGS. 7 and 8, the proximal end of cannula 14includes two slots 72 and 74, preferably separated by at least90.degree. Alignment slot 72 serves to maintain rotational alignment ofcannula 14 with respect to cannula housing 21. Referring additionally toFIGS. 9 and 10, alignment pin 76 is fixedly secured within aperture 78in lower cannula housing 80 and, when fully inserted into housing 80,extends into lumen 82. In assembling cannula 14 within lower cannulahousing 80, alignment pin 76 is positioned within alignment slot 72,thereby preventing rotation of cannula 14 yet permitting axial movementof cannula 14 relative to cannula housing 21. Additionally, inwardlydirected orientation pin 84 on proximal housing extension face 86cooperates with an orientation slot 88 in bottom housing 90 (see FIG.11) to ensure proper alignment between cannula housing 21 and trocarhousing 20. A gasket 92 and stabilizer plate 96 are positioned withinflange 94 in lower cannula housing 80 to provide a gas seal withinserted instrumentation, and to cooperate with an internal flappervalve, as is known in the art.

[0045] Slot 74 in cannula 14 forms an internal shelf 98 with which latch100 (see FIG. 14) is adapted to engage. Internal shelf insert 102 (FIGS.12 and 13) is positional within cavity 104 in lower cannula housing.Shelf insert 102 comprises angled latching faces 106, bridging arm 108and longitudinal slot 110. Shelf insert 102 is positioned within cavity104 such that angled latching faces 106 are directed distally and edges112 are substantially aligned with internal shelf 98 of cannula 14.Bridging arm 108 abuts outer wall 114 of cavity 104 and is typicallysecured thereto, e.g., by an adhesive.

[0046] Latch 100 has a body 116 which forms a latch finger 118 whichincludes an outer camming face 120 and an inner latching face 122.Aperture 124 is located in mid-region 117 of body 116 and permits latch100 to be movably joined to latch release finger 126 (see FIGS. 15 and17). Mid-region 117 is of reduced thickness relative to the remainder ofbody 116 to accommodate latch release finger 126. Aperture 128 is formedin the region of body 116 distant from latch finger 118.

[0047] Latch release finger 126 includes a substantiallytriangularly-shaped extension 138 having distal and proximal cam faces140 and 142, respectively. Latch release finger 126 also includes anaperture 144 and a spring abutment region 145. As shown in FIG. 16,latch release finger 126 is movably mounted to latch 100 by means of apin (not pictured) which passes through apertures 144 and 124 to form alatch subassembly 147. When release finger 126 is positioned such thatextension 138 is directed toward aperture 128, abutment region 145extends below latch wall 148.

[0048] Latch subassembly 147 is pivotally joined to obturator shaft 22by a pin (not pictured) which passes through aperture 128 in latch body116 and aperture 132 in hollow region 130 of shaft 22. As shown in FIG.16, a leaf spring 134 is mounted to shelf 136 in hollow region 130,e.g., by an adhesive. The proximal region 146 of leaf spring 134 biaseslatch 100 clockwise around aperture 128 and biases release finger 126counter-clockwise relative to aperture 144. Hollow region 130 is sizedand dimensioned to permit latch subassembly 147 to be fully recessedtherewithin (against the bias of leaf spring 134). Inasmuch as releasefinger 126 is restricted in its counter-clockwise rotation by latchfinger 118 and/or abutment with wall 119, and in its clockwise rotationby engagement between spring abutment region 145 and leaf spring 134,the full range of rotation of release finger 126 is approximately90.degree.

[0049] In use, and referring generally to FIGS. 18-21 cutting tip 12 andobturator shaft 22 are introduced through cannula housing 21 and intocannula 14. When obturator shaft 22 is fully inserted, trocar housing 16abuts cannula housing 21. However, cutting tip 21 remains within cannula14 until trocar 10 is armed by the surgeon. To arm the instrument, thesurgeon advances button 40 within channel 42 which distally advancesobturator shaft 22 and causes cutting tip 12 to extend beyond cannula14. Distal movement of obturator shaft 22 also results in distalmovement of latch subassembly 147.

[0050] Leaf spring 134 biases latch subassembly 147 outward from hollowregion 130. As obturator shaft 22 moves distally, outwardly biased latchsubassembly 147 passes through aperture extension 60 in spring-retainingplate 54. Outer camming face 120 of latch 100 then contacts flange 56 atthe proximal end of cannula 14, causing counterclockwise rotation oflatch subassembly 147 relative to aperture 128 (see FIG. 17). Thiscounterclockwise rotation recesses latch subassembly 147 within hollowregion 130 against the bias of leaf spring 134.

[0051] As cutting tip 12 approaches its fully armed position, latchsubassembly 147 comes into alignment with slot 74 in cannula 14. Outercamming face 120 is thus freed from contact with the inner surface ofcannula 14, and leaf spring 134 causes latch subassembly 147 to rotateclockwise such that latch finger 118 extends radially outward fromhollow region 130. However, latch release finger 126 remains positionedsuch that extension 138 is directed substantially toward aperture 128,with abutment region 145 resting against leaf spring 134. The freeing ofcamming face 120 from contact with cannula 14 is generally accompaniedby an audible click, signalling the surgeon that trocar 10 is armed andfurther distal movement of button 40 is unnecessary.

[0052] When the surgeon releases button 40, tension spring 38 drawsobturator shaft 22 proximally until inner latching face 122 of latch 100engages internal shelf 98 and shelf insert 102. Latch release finger 126passes back within cannula 14 proximal of slot 74, thereby pressingabutment region 145 against the bias of leaf spring 134. In this fullyarmed position of FIGS. 1 and 2, cutting tip 12 extends beyond cannula14 and button 40 is distally located within slot 42.

[0053] The surgeon presses armed trocar 10 against the body wall of apatient, thus causing piercing tip 12 to incise the tissue. As cuttingtip 12 passes through the body wall, the distal end of cannula 14 isbrought into engagement with tissue. The counterforce exerted by thebody wall against cannula 14 and abutment surface 15 causes cannula 14to move proximally against the bias of compression springs 68. Thisproximal movement of cannula 14 release latch release finger 126 toenter slot 74. Latch release finger 126 rotates counterclockwiserelative to aperture 144 within longitudinal slot 110 in shelf insert102.

[0054] As the surgeon continues to press trocar 10 against the bodywall, piercing tip 12 enters the body cavity. Continued pressure by thesurgeon advances cannula 14 into the body cavity as well. As soon as thecounterforce of the body wall against the distal end of the cannula 14and abutment surface 15 is surpassed by the distally-directed force ofcompression springs 68 against spring-retaining plate 54, cannula 14 isdriven distally relative to cannula housing 18. As cannula 14 movesdistally, internal shelf 98 contacts proximal cam face 142 of latchrelease finger 126. Latch release finger 126 is thus drivencounterclockwise such that extension 138 abuts latch finger 118 and/orthe body of latch release finger 126 abuts wall 119 of latch 100. Inthis position, latch release finger 126 prevents engagement betweeninternal shelf 98 and latch finger 118.

[0055] As cannula 14 continues to move distally, internal shelf 98 camslatch subassembly 147 counterclockwise relative to aperture 128 againstthe bias of leaf spring 134, thus freeing latch finger 118 fromengagement with shelf insert 102. As soon as the tip of latch finger 118rotates out of engagement with shelf insert 102, tension spring 28 drawsobturator shaft 22 and cutting tip 12 proximally such that cutting tip12 is positioned within cannula 14. Button 40 is also drawn proximallywithin slot 42 and is once again positioned to allow the surgeon to armtrocar 10, if so desired.

[0056] If, after arming trocar 10, the surgeon determines that it isdesirable to manually retract cutting tip 12 into cannula 14, thesurgeon simply moves cannula 14 proximally with respect to cannulahousing 18 and releases. Cannula 14 will then move distally under thebias of compression springs 68 rotating latch subassembly 147counterclockwise. Latch finger 118 is thus moved out of engagement withshelf insert 102, allowing tension spring 28 to withdraw cutting tip 12into cannula 14.

[0057] The position of button 40 within slot 42 provides the surgeonwith a visual indication of the position of cutting tip 12 relative tocannula 14. The audible click associated with the movement of latch 100during the arming of trocar 10 also provides an aural signal to thesurgeon.

[0058] Many structures may be included toward the distal end of cannula14 to facilitate the sensing of body wall counterforce to effectuatereciprocation of cannula 14, abutment surface 15 being but one example.Exemplary structures include radially spaced, outwardly directedprotuberances, inflatable means of the type known in the art for fixedlypositioning catheters and like devices, and flange means of varyinggeometries. Such structures may be fixedly secured to cannula 14 orrepositionable along the longitudinal axis of cannula based on suchfactors as patient size and weight.

[0059] While the above description contains many specific details, thesedetails should not be construed as limitations on the scope of theinvention, but merely as examples of preferred embodiments thereof.Those skilled in the art will envision many other possible variationsthat are within the scope and spirit of the invention as defined by theclaims appended hereto.

What is claimed is:
 1. A trocar comprising: (a) a cannula assemblycomprising a cannula and a cannula housing defining an internal cannulapassage, said cannula being reciprocally mounted with respect to saidcannula housing for movement between a distal position and a proximalposition; (b) a trocar assembly adapted to cooperate with said cannulaassembly, said trocar assembly comprising a piercing tip mounted to anobturator shaft and a trocar housing, said piercing tip and at least aportion of said obturator shaft being configured and dimensioned forentry into said internal cannula passage; (c) releasable obturator meansassociated with the obturator shaft which releasably maintains thepiercing tip in an extended position relative to said cannula; (d)release means associated at least in part with the cannula assembly andactuated by movement of said cannula with respect to said cannulahousing for releasing the releasable obturator means; and (e) biasingmeans for retracting said piercing tip and said obturator shaft fromsaid extended position to a retracted position in response to release ofthe releasable obturator means.
 2. The trocar of claim 1, wherein saidcannula includes a slot which defines an internal shelf and saidinternal shelf is adapted to cooperate with said releasable obturatormeans to maintain said piercing tip in said extended position.
 3. Thetrocar of claim 1, wherein said releasable obturator means comprises alatch and a spring biasing said latch radially outward.
 4. The trocar ofclaim 1, wherein said release means for releasing said releasableobturator means comprises a slot in said cannula.
 5. The trocar of claim4, wherein said release means further comprises a latch release fingerassociated with said releasable obturator means and said slot in saidcannula is adapted to align with said latch release finger.
 6. thetrocar of claim 1, wherein said trocar assembly further comprises anindicator means which signals the position of said piercing tip.
 7. Thetrocar of claim 1, wherein said release means for releasing thereleasable obturator means and said biasing means cooperate toautomatically retract said piercing tip to said retracted position inresponse to removal of a counterforce from the distal end of saidcannula.
 8. The trocar of claim 1, wherein said cannula assembly furthercomprises cannula biasing means.
 9. The trocar of claim 8, wherein saidcannula biasing means comprises at least one spring biasing said cannulatoward said distal position.
 10. The trocar of claim 1, wherein saidcannula includes an abutment means.
 11. The trocar of claim 10, whereinsaid abutment means is positioned on the exterior of said cannula towardthe distal end thereof.
 12. The trocar of claim 10, wherein saidabutment means is selected from the group consisting of acircumferential abutment surface, a plurality of radially spaced,outwardly directed protuberances, inflatable means, and flange means.13. The trocar of claim 10, wherein said abutment means isrepositionable along the longitudinal axis of said cannula.
 14. Thetrocar of claim 1, wherein said releasable obturator means comprises alatch and a leaf spring.
 15. The trocar of claim 14, wherein saidrelease means comprises a latch release finger and said latch isrotatably joined to said latch release finger by pin means.
 16. Thetrocar of claim 1, wherein said obturator shaft includes a hollow regionin which is mounted a leaf spring and a latch subassembly, said leafspring biasing said latch subassembly radially outward.
 17. The trocarof claim 1, wherein said release means for releasing said releasableobturator means comprises a shelf insert, said shelf insert forming alongitudinal slot.
 18. A trocar comprising: (a) a cannula assemblycomprising a cannula reciprocally mounted to a cannula housing, saidcannula defining an internal shelf; (b) a trocar assembly comprising apiercing tip, an obturator shaft and a trocar housing; (c) latch meansassociated with said obturator shaft, said latch means adapted toreleasably engage said internal shelf; and (d) biasing means forretracting said piercing tip from an extended position wherein saidpiercing tip extends beyond said cannula to a retracted position whereinsaid piercing tip is shielded by said cannula; wherein reciprocation ofsaid cannula with respect to said trocar assembly releases said latchmeans from engagement with said internal shelf and allows said biasingmeans to retract said piercing tip to said retracted position.
 19. Thetrocar of claim 18, wherein said latch means comprises a latch and aleaf spring abutting said latch.
 20. The trocar of claim 18, whereinsaid cannula assembly further comprises a shelf insert positionedadjacent said internal shelf, said latch means engaging both saidinternal shelf and said shelf insert.
 21. The trocar of claim 18,wherein said cannula may be reciprocated manually by a surgeon orthrough contact with a body wall of a patient.
 22. The trocar of claim11, wherein said cannula assembly further comprises abutment means onsaid cannula.
 23. A method for inserting a trocar, comprising: (a)advancing an obturator shaft and a piercing tip to expose said piercingtip from a cannula, said piercing tip being maintained in said exposedposition through engagement of latch means associated with saidobturator shaft with a shelf formed in said cannula; (b) pressing saidpiercing tip against a body wall, said body wall thereby exerting acounterforce against said cannula and causing reciprocation of saidcannula into a cannula housing against the bias of cannula biasingmeans; (c) entering said piercing tip into a body cavity, therebyremoving said counterforce from said cannula, said cannula biasing meanscausing said cannula to move distally, thereby releasing said latchmeans from engagement with said shelf, whereby said piercing tip isautomatically withdrawn into said cannula under the action of obturatorbiasing means.
 24. The method of claim 23, wherein said latch meanscomprises a latch and a leaf spring abutting said latch and biasing saidlatch radially outward.
 25. The method of claim 23, wherein said cannulaincludes abutment means which facilitate sensing of counterforce by saidcannula.
 26. A trocar comprising: (a) a cannula assembly comprising acannula and a cannula housing defining an internal cannula passage; (b)a trocar assembly adapted to cooperate with said cannula assembly, saidtrocar assembly comprising a trocar housing and obturator means whichincludes a piercing tip at a distal end thereof, at least a portion ofsaid obturator means being configured and dimensioned for entry intosaid internal cannula passage; (c) button means mounted to saidobturator means and extending from said trocar housing for advancingsaid piercing tip to an extended position relative to said cannula; (d)latching means mounted to said trocar assembly and adapted to cooperatewith engagement means, said latching means adapted to assume a latchedposition wherein said latching means engages said engagement means andmaintains said piercing tip in said extended position relative to saidcannula, and an unlatched position wherein said latching means isreleased from engagement with said engagement means; and (e) biasingmeans for retracting said piercing tip from said extended position to aretracted position after said latching means has assumed said unlatchedposition.
 27. The trocar of claim 26, wherein said engagement meanscomprises a shelf formed on said cannula.
 28. The trocar of claim 26,wherein said latching means is mounted to said obturator means.